1. Field of the Invention
The present invention relates to an improved method and apparatus for grinding a workpiece having a cylindrical shape.
2. Discussion of the Prior Art
Heretofore, in a conventional cylindrical grinding machine, a wheel head supporting a rotating grinding wheel 19 is advanced toward a workpiece W, so that an outer peripheral surface of the workpiece W which is supported between a center 15a of a work head and a center 16a of a tailstock, is ground with the grinding wheel 19, as shown in FIG. 1. As shown in FIG. 2, the wheel head is infed as reducing its infeed rate in order such as at a rough grinding infeed rate G1, a fine grinding infeed rate G2 and a finish grinding infeed rate G3. Therefore, the workpiece W is ground with the grinding wheel 19, wherein the position of the wheel head is represented by the solid-line G. Further, to compare the position of the wheel head with the diameter of a workpiece portion Wa of the workpiece W, the diameter of the workpiece portion Wa is converted into a corresponding value which is represented as the position of the wheel head. The corresponding value decreases as indicated by the broken-line H as the grinding operation proceeds. In such a grinding operation, for higher accuracy, the machining is carried out with the diameter of the workpiece W being measured by an in-process measuring device. Namely, when the diameter of the workpiece portion Wa reaches a rough grinding end diameter d1, the infeed rate is changed from the rough grinding infeed rate G1 to the fine grinding infeed rate G2 and then, when the diameter thereof reaches a fine grinding end diameter d2, the infeed rate is changed from the fine grinding infeed rate G2 to the finish grinding infeed rate G3. Further, when the diameter reaches a finish diameter D, the grinding operation comes to end and the wheel head is retracted.
However, in the prior art grinding apparatus, the grinding operation causes the workpiece and supporting member thereof to be bent, whereby the workpiece has an amount left not ground. This amount is called residual grinding amount hereinafter throughout the specification. As represented by the solid-line I in FIG. 2, the residual grinding amount at each of the grinding steps is saturated to a certain value with the progress of the operation. Particularly, it is required that the residual grinding amount is saturated to the certain value upon completion of the finish grinding step, for a desired surface roughness and a desired roundness of the surface. Further, although the sharpness of the grinding wheel is high immediately after the truing operation, it is gradually deteriorated as the grinding amount of the workpiece increases. This causes the cycle time for grinding one workpiece to change to a relatively large extent. In the case that a machining line having plural machine tools connected in line machines the workpieces, if there occurs such change of the machining cycle time in at least one of the plural machine tools, then the cycle time of the entire machining line is adversely affected, whereby the efficiency of the machining line is lowered. Therefore, the change of the machining cycle time has to be eliminated.
The change of the machining cycle time is largely contributed by that in the finish grinding step (if any, the fine grinding step prior thereto) with a slow infeed rate. Therefore, the machining cycle time change could be diminished if an attempt is made to shorten the time required for the finish grinding step (and the preceding fine grinding step), in other words, to make the workpiece diameter at the end of rough (or the successive fine) grinding step be approximated to a finish diameter.
However, in this manner, the residual grinding amount cannot be saturated during the finish grinding step, thereby resulting in low degrees of the surface roughness and roundness of the ground surface.